The potential for property damage, personal injury and death from hostile fires in buildings increases in proportion to the time it takes to detect the fire. Put another way, the earlier a hostile fire can be detected in a building the less severe will be its consequences.
More sensitive smoke detectors will detect a fire more quickly than less sensitive smoke detectors. In a controlled test environment, described in Underwriter's Laboratories standard 268, a detector set at 0.5% obscuration per foot will repeatedly detect smoke in the "smoldering fire room test"14 to 17 minutes faster than the same detector set at 3.7% obscuration per foot. More sensitive smoke detectors are, however, more prone to false alarms than less sensitive detectors. The challenge, then, is to match the individual detector sensitivity to the environment where it is installed so as to minimize the time to detect a fire and to do so without causing false alarms.
The Simplex TrueAlarm.TM. smoke detection system disclosed in U.S. Pat. No. 5,155,468 allows a user to set the sensitivity of each detector in the system. Sensitivity is determined by a threshold value above which an alarm is sounded, that threshold being computed from a user selected delta threshold added to a normal base level. In the TrueAlarm system, that normal base level is computed as a running average of the background sensor signal.
The system has the ability to display, on command from the system's history log, the actual maximum percent of the alarm level experienced as background at every smoke sensor location since the history log for the sensor was last reset. For example, if a smoke detector system monitoring a particular environment were programmed to a 3% per foot sensitivity and, at some time during the period the sensor was being monitored, actual smoke in that environment reached 1% per foot obscuration, the panel display would show that the maximum actual value for that sensor during that time period reached 33% of its alarm setpoint.
Thus, a user may periodically check the maximum percent and adjust the sensitivity of the detector accordingly. For example, if the maximum percent over a period of time is very close to the alarm threshold, one might conclude that the environment naturally provides sensor signals which are too close to the present threshold. The sensitivity of the detector would be reduced to avoid false alarms by increasing the delta threshold. On the other hand, if the maximum percent is always very low, the sensitivity could be increased for more rapid response to a fire without risking false alarms.